METALS AND CYANIDE PROCESSES 231 



New Zealand Crown mine, in June and July, 1888. Com- 

 mercial success dales from the introduction of the MacArthur- 

 Forrest process, in 1890, in the Witwatersrand gold fields 

 in South Africa, the first cyanide plant in the world for treat- 

 ing tailings having been erected at Johannesburg in April, 

 1890. In the Witwatersrand alone, at the end of 1891, there 

 were 6 companies treating taiUngs by the cyanide process; 

 at the end of 1892 there were 22; and at the end of 1893 there 

 were 32, with a record of 143,500 tons per month treated. 

 By the use of this process there were recovered in the Rand 

 286 ounces of gold in 1890, 34,862 ounces in 1891, 178,688 

 ounces in 1892, 330,510 ounces in 1893, 714,122 ounces in 

 1894, 753,490 ounces in 1895, and 703,704 ounces in 1896; 

 the output then increased up to September, 1898, when the 

 commencement of active hostilities in the Boer war inter- 

 fered with the active working of the mines. 



The cyanide process consists in lixiviating the finely 

 powdered ore with a dilute solution of an alkali qyanide, 

 drawing off this solution when charged with the precious 

 metals, and precipitating these metals from the solution. 

 In the patents of MacArthur and Forrest the claims were 

 made for the use of dilute solutions of cyanide (not more than 

 8 parts of cyanogen to 1,000 parts of water) ; the emploj^ment 

 of caustic alkalies for neutralizing acid ores, prior to their 

 digestion in the cyanide solution; and the use of zinc, pref- 

 erably in a filiform condition, as a precipitant. The cyanide 

 first used was potassium cyanide, but cj'an-salt, which is a 

 mixture of sodium and potassium cyanides, has come into 

 extended use. The tailings, or a charge of ore crushed to 

 the desired fineness, are placed in the leaching vats or tanks 

 and, if acid, given a preliminary treatment with lime or sodium 

 hydroxide, which is generally washed out before further 

 treatment. The ore is then subjected to the action of the 

 cyanide solution, which is usually admitted at the bottom 

 of the leaching vat; after digesting for a length of time, de- 

 pending on the character of the charge, its fineness, its free- 

 dom from slimes, and the strength of the cyanide solution, 

 the solution containing the precious metals is draT\Ti off and 

 the operation repeated. 



